Photonic crystal (PC) material are materials having a periodic modulation in their refractive index (Yablonovitch, Phys. Rev. Lett., 58:2059, 1987), giving rise to a photonic band gap or stop gap, in which electromagnetic waves within a certain stop band wavelength range are totally or mostly reflected. The wavelengths of the stop band are dependent on the distance between the periodic modulations in the material. The reflected stop band wavelengths appear in the reflectance spectrum as a distinct reflectance peak that may be referred to as a Bragg peak. The crystal may have a one-, two-, or three-dimensional periodic structure.
Because of the sensitivity of a PC material, slight changes in the refractive index and/or lattice spacing of the material may result in detectable changes in the reflected light. This may be particularly useful where the reflected light is in the visible range, allowing for detectable changes in color if the refractive index or lattice spacing is modulated. By incorporating polymers into PC materials, these materials may be made responsive to external stimuli, such as mechanical forces. An example of such an application is described by Arsenault et al. in PCT Patent Application No. 2008/098339, which is herein incorporated by reference in its entirety.
Photonic crystal materials have been used in security articles to indicate authenticity, because of their reflective nature and their property of displaying color-shift upon tilting the material relative an incident light source or to a viewer. Despite the attractiveness of photonic crystal materials for use in security devices, there has been an unmet need for relatively rapid and cost-effective patterning or printing of these materials.